About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Shelf Sands and Sandstones — Memoir 11, 1986
Pages 241-255
Storm-Dominated Shelves - Products

Proximal and Distal Hummocky Cross-Stratified Facies on a Wide Ordovician Shelf in Iberia

Patrick J. Brenchley, Michael Romano, Juan Carlos Gutierrez-Marco

Abstract

The sandstone of the Monte da Sombadeira Formation (25 m to 200 m thick) crops out repeatedly on the flanks of open folds in an area 550 km long and 140 km wide (approximately 75 000 km2). At all localities there is a vertical change from mudstone to fissile siltstone into the sandstone of the formation itself and a return to fissile siltstone, then mudstone above the formation. Within the sandstone formation there are lateral facies differences, which are related to the paleogeography of the Ordovician shelf.

Proximal localities have an interbedded mudstone/laminated siltstone facies with wave ripples and small hummocks. The associated sandstone facies has hummocky cross-stratification, which is characterized by: 1. deep scours at the bases of the beds; 2. high angles of dip to the hummocky cross-stratification; 3. deep scours at the tops of the beds; 4. wave ripples at the tops of the beds; and 5. beds that are variable in thickness and are often discontinuous. In contrast, distal localities have: 1. shallow erosion at the bases of beds; 2. low angle hummocky cross-stratification; 3. shallow erosion at the tops of beds; 4. rare or no wave ripples; and 5. beds that may be variable in thickness, but arc rarely discontinuous. Amalgamation of sandstone beds is found in the most proximal and distal localities but is less common in between.

From the study we conclude: 1. the sandstone beds were emplaced during a regressive phase; 2. sandstone beds (up to 50 cm thick) with hummocky cross-stratification were deposited more than 100 km from the shore; 3. turbidite-like sandstones were not formed; 4. parallel lamination is the only current-formed structure preserved and wave-formed structures predominate generally throughout the sequence; and 5. it is possible in this case to distinguish between proximal and distal facies with hummocky cross-stratification. We estimate the average seaward dip of the Ordovician Iberian shelf was less than 0.1°, and we believe the low gradient had an important influence in determining the facies, by inhibiting the strength of offshore directed currents.


Pay-Per-View Purchase Options

The article is available through a document delivery service. Explain these Purchase Options.

Watermarked PDF Document: $14
Open PDF Document: $24